1. Field of the Invention
The present invention relates to a continuously variable automatic transmission mounted in, for example, a vehicle. More particularly, the present invention relates to a continuously variable automatic transmission provided with a forward-reverse switching mechanism disposed between a stepless speed change unit and an output shaft in a power transmitting path. The forward-reverse switching mechanism includes a fixed (always stationary) element of a gear mechanism, a clutch which is engaged in forward, and a clutch which is engaged in reverse.
2. Description of the Related Art
A belt type (CVT type) continuously variable automatic transmission (hereinafter referred to as a “continuously variable transmission”) mounted in, for example, a vehicle, typically includes a forward-reverse switching mechanism for switching the input rotation between forward rotation and reverse rotation, and a speed change unit for freely changing the speed ratio. The forward-reverse switching mechanism and the stepless speed change unit can be arranged between an input shaft which is rotatably driven by an engine, and an output shaft (wheel axles) which is connected to the vehicle drive wheels, with the forward-reverse switching mechanism being positioned either in front of the stepless speed change unit or in the rear of the stepless speed change unit.
The preferred order of arrangement is: input shaft, forward-reverse switching mechanism, stepless speed change unit, and output shaft (see JP(A) 61-270542), which arrangement enables the forward-reverse switching mechanism to be made relatively compact. Further, this preferred arrangement prevents a large amount of torque from being applied to the forward-reverse switching mechanism because the rotation received by the forward-reverse switching has been slowed by the stepless speed change unit. However, in this arrangement, because the stepless speed change unit is connected to the vehicle drive wheels, if the vehicle stops suddenly, for example, the pulley of the stepless speed change unit stops, together with the vehicle drive wheels, while on the high speed side, which may result in the vehicle being unable to take off again. Also, if the vehicle is towed while the engine is running, no lubrication oil is circulated and the durability of the stepless speed change unit may be adversely affected due to the fact that it is rotated by the vehicle drive wheels.
Therefore, it has been proposed to cut off the transmission of power between the stepless speed change unit and the output shaft. For example, JP(A) 2001-124191 discloses a drivetrain having a clutch between the stepless speed change unit and the output shaft. JP(A) 11-159596 discloses a drivetrain with the input shaft, the stepless speed change unit, the forward-reverse switching apparatus, and the output shaft arranged in this order. As a result, even if the vehicle (i.e., the vehicle drive wheels) is stopped, the pulley of the stepless speed change unit can be returned to the low speed side while being rotatably driven, and rotation of stepless speed change unit can be prevented when the vehicle is being towed.
Typically, the forward-reverse switching mechanism outputs reverse rotation by engaging a brake to stop rotation of one element of a planetary gear set and rotating a gear in reverse relative to the input rotation. Because input torque and torque that becomes a reaction force act on that brake, it receives application of a larger amount of torque than that applied to a clutch which selectively transmits power.
In a continuously variable transmission in which the input shaft, the stepless speed change unit, the forward-reverse switching mechanism, and the output shaft are arranged in this order in the power transmitting path, rotation is input to the forward-reverse switching mechanism only after it has been slowed by the stepless speed change unit. As a result, a relatively large amount of torque is input to the forward-reverse switching mechanism, particularly when the vehicle starts moving, so the brake must withstand an even larger amount of torque and must be made correspondingly larger, so that the objective of compactness of the continuously variable transmission cannot be achieved.
Further, provision of a clutch between the stepless speed change unit and the output shaft, as described above, means the addition of another clutch, which creates additional problems. For example, the additional clutch hinders the objectives of compactness and cost reduction due to the increase in the number of components, and also increases the complexity of control.